Loosen grip and a $10 cramp buster worked for me. Used to have that problem after a couple hours on my throttle hand (since it rarely gets to rest), but the cramp buster allows the palm of your hand to rest on it changing the angle and relieving pressure. There are better long term options (as mentioned, bars, risers, etc, or you can go with a cruise control, but that won't help for fun riding), but give the $10 fix a try and see what you think!